532
FXUS65 KBYZ 211035
AFDBYZ
AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE BILLINGS MT
435 AM MDT THU MAY 21 2026
KEY MESSAGES
- COOL AND WET WEATHER SYSTEM BRINGS LOWER ELEVATION RAIN AND
MOUNTAIN SNOW TODAY; EXPECT SCATTERED THUNDERSTORMS CAPABLE OF
PRODUCING SMALL HAIL AND LOCALIZED DOWNPOURS THIS AFTERNOON AND
EARLY EVENING NEAR THE DAKOTAS BORDER.
- PRECIPITATION AMOUNTS OF 0.50-1.00" (LOCALLY HIGHER) OVER
SOUTHERN MOUNTAINS AND FOOTHILLS.
- HEAVY SNOW ON NORTH ASPECTS OF THE BEARTOOTH MOUNTAINS MAY
IMPACT THE OPENING OF THE BEARTOOTH HIGHWAY.
- WARMING TREND AND MOSTLY DRY FRIDAY THROUGH MONDAY; EXPECT
MOUNTAIN SNOW MELT TO INCREASE RIVER FLOWS.
- NEXT WEATHER SYSTEM MAY BRING INCREASED THUNDERSTORM ACTIVITY BY
NEXT TUESDAY AND WEDNESDAY.
UPDATE
WIDESPREAD PRECIPITATION CONTINUES OVER THE AREA EARLY THIS
MORNING AS AN UPPER TROUGH IS MOVING SLOWLY OVERHEAD, WITH AN AREA
OF FRONTOGENESIS CURRENTLY SITUATED FROM NORTH OF MILES CITY TO
NEAR PRYOR/RED LODGE. RAIN IS SITUATED ALONG THE AREA OF
FRONTOGENESIS, AND ADDITIONAL PRECIPITATION IS SURGING IN FROM THE
SOUTH AND WEST AS THE TROUGH PIVOTS. AREAS SOUTH AND WEST OF
BILLINGS TOWARDS THE FOOTHILLS HAVE PICKED UP AS MUCH AS .75
INCHES ALREADY, WITH RADAR ESTIMATES SHOWING CLOSER TO AN INCH IN
SOME AREAS. EXPECT THE RAIN/MOUNTAIN SNOW TO CONTINUE THIS
MORNING, SLOWING SHIFTING SOUTH AND EAST THROUGH THE DAY, BECOMING
MORE SHOWERY IN THE AFTERNOON. WEAK THUNDERSTORMS ARE POSSIBLE AS
WELL, WITH THE GREATEST POTENTIAL NEAR THE DAKOTAS BORDER. HAVE
ADJUSTED TEMPERATURES AND POPS TO LATEST TRENDS. OTHERWISE, HAVE
ONLY MADE MINOR ADJUSTMENTS TO THE PRECIPITATION FORECAST, AS THAT
STILL REMAINS IN GOOD SHAPE. STP
DISCUSSION
THROUGH NEXT WEDNESDAY...
DYNAMIC UPPER TROF MOVING SLOWLY THRU WESTERN MT, SOUTHEAST ID AND
WY WILL BRING WIDESPREAD PRECIPITATION THROUGH THIS EVENING,
INCLUDING HEAVY MOUNTAIN SNOW ON NORTH ASPECTS ABOVE 7KFT. ALL OF
THIS IS COVERED WELL BY THE CURRENT FORECAST INCLUDING WINTER
HIGHLIGHTS FOR THE BEARTOOTH-ABSAROKA, PRYOR & BIGHORN MOUNTAINS.
EXPECT PRECIP TO TAPER OFF IN THE WEST BY MIDDAY (SCATTERED LIGHT
SHOWERS STILL POSSIBLE ON THE BACK SIDE OF THE TROF UNDER THE
COOL AIR ALOFT), CENTRAL PARTS BY EVENING, AND THE EAST SOMETIME
LATE TONIGHT OR EARLY FRIDAY. GREATEST PRECIP AMOUNTS WILL BE IN
OUR SOUTHERN UPSLOPE AREAS, 0.75-1.00", WITH A 30% CHANCE OF
EXCEEDING AN INCH ALONG THE IMMEDIATE NORTH ASPECTS. REGARDING
SNOWFALL, LOOK FOR 8-15" IN THE BEARTOOTH-ABSAROKAS AND 5-10" OVER
THE PRYORS AND BIGHORNS. SNOW WILL IMPACT CONDITIONS OVER THE
BEARTOOTH HIGHWAY (SCHEDULED TO OPEN FRIDAY) AS WELL AS US-14
THROUGH THE BIGHORNS.
A FEW SMALLER DETAILS YET TO BE WORKED OUT. THERE WILL BE A REGION
OF MID LEVEL FRONTOGENESIS PROPAGATING WEST-TO-EAST THAT SHOULD
BE A FOCUS OF HEAVIEST PRECIP AS IT PASSES. AS A RESULT, LOOK FOR
AN AREA OF ENHANCED PRECIP EXTENDING NE OF OUR SOUTHERN UPSLOPE
AREAS, WITH ENSEMBLES SUGGESTING IN A GENERAL SENSE THIS WILL
OCCUR FROM NEAR PRYOR TO FORSYTH-ISH. RAINFALL AMOUNTS IN EXCESS
OF A HALF INCH ARE LIKELY TO OCCUR WHERE THIS FRONTOGENESIS
LINGERS LONGEST. SOMETHING TO WATCH. NEXT, WHILE BURN SCARS WILL
NEED TO BE WATCHED, RAINFALL RATES (0.10-0.20"/HR) ARE NOT
EXPECTED TO BE GREAT ENOUGH TO CAUSE RUNOFF PROBLEMS. ALSO, SNOW
LEVELS (ABOUT 7KFT AT 05Z PER OBS AND MESOANALYSIS) ARE EXPECTED
TO FALL TO NEAR 6KFT BY 12Z. THIS PUTS SNOWFALL ABOVE RED LODGE
AND STORY BUT NOT BY MUCH. IT IS POSSIBLE THAT THRU ADDITIONAL
DYNAMIC COOLING WE SEE A BRIEF PERIOD OF WET SNOW MAKE IT DOWN TO
NEAR THESE LOCATIONS...BUT EVEN MINOR ACCUMULATION IS NOT LIKELY.
FINALLY, WITH THE SURFACE TROF LINGERING NEAR THE DAKOTAS BORDER
AT MIDDAY AND MOISTURE RETURN TO ITS EAST, THERE IS A RISK OF
T-STORMS IN OUR FAR EAST IN THE AFTERNOON, AHEAD OF THE MAIN AREA
OF PRECIP. WITH CAPES OF PERHAPS SEVERAL HUNDRED J/KG, 0-6KM BULK
SHEAR UP TO 25KTS, AND PWATS OF 0.75", STORMS WOULD SEEM TO HAVE A
CHANCE OF PRODUCING UP TO HALF INCH HAIL AND LOCALIZED DOWNPOURS.
THAT SAID, THERE IS OVERWHELMING MODEL CONSENSUS FOR THE GREATER
INSTABILITY TO BE IN THE WESTERN DAKOTAS.
DESCENT SPREADS IN FROM WEST TO EAST BEHIND THE TROF LATE TODAY
AND TONIGHT, WITH PERHAPS SOME LIGHT SHOWERS LINGERING IN OUR FAR
EAST INTO FRIDAY MORNING. TOMORROW WILL BE DRIER & WARMER WITH
TEMPS BACK TO THE 60S (AFTER MID 40S TO NEAR 60F TODAY). A WEAK
SHORTWAVE WITHIN THE INCREASINGLY ZONAL FLOW MAY BE ENOUGH TO
PRODUCE WEAK DIURNAL CONVECTION IN THE AFTERNOON AND EARLY
EVENING. WARMING TREND CONTINUES THIS WEEKEND UNDER A BUILDING
RIDGE. LOOK FOR 70S SATURDAY AND 80S SUNDAY. AGAIN, THESE WILL BE
MOSTLY DRY DAYS BUT CANNOT RULE OUT SOME WEAK DIURNAL SHOWERS
ESPECIALLY CLOSE TO THE MOUNTAINS.
FLOW BACKS TO THE SW MONDAY AHEAD OF THE NEXT PACIFIC TROF, AND WE
COULD SEE TEMPS NEAR 90F ON MEMORIAL DAY. THEREAFTER THINGS GET
SKETCHIER. CLUSTER ANALYSES CONTINUE SHOW GREAT SPREAD IN
SOLUTIONS REGARDING THE TROF'S TRACK THROUGH THE WESTERN CONUS,
BOTH IN TERMS OF TIMING AND LOCATION. FOR NOW, LOOK FOR GRADUAL
COOLING AND INCREASING CHANCES OF SHOWERS/T-STORMS TUESDAY &
WEDNESDAY. WE SHOULD SEE SOME DEGREE OF MOISTURE RETURN UP THE
HIGH PLAINS AHEAD OF THE LOW, AND IF THINGS COME TOGETHER WE COULD
SEE A RISK OF SEVERE T-STORMS AT SOME POINT. IT IS ALSO POSSIBLE
THAT WE SEE ANOTHER PRECIP EVENT IF THE LOW TAKES A FAVORABLE
TRACK. ALL THIS IS OF LOW CONFIDENCE AT THIS TIME.
JKL
AVIATION
WIDESPREAD RAIN AND MOUNTAIN SNOW WILL CONTINUE TODAY, GRADUALLY
SHIFTING SOUTH AND EAST THROUGH THE AFTERNOON AND EVENING. LOOK
FOR STRATIFORM RAIN TO BECOME MORE SHOWERY THIS AFTERNOON, WITH
ISOLATED TO SCATTERED THUNDERSTORMS POSSIBLE (15-25% CHANCE) OVER
THE FAR WEST AND THE FAR EAST, INCLUDING KBHK. THE
SHOWERS/THUNDERSTORMS WILL LINGER LONGEST OVER THE FAR EAST,
POTENTIALLY INTO FRIDAY MORNING. MVFR TO IFR CONDITIONS ARE
POSSIBLE WITH THE RAIN, WITH CONDITIONS GRADUALLY IMPROVING
TOWARDS VFR AS THE PRECIPITATION DIMINISHES/SHIFTS EAST. MOUNTAIN
OBSCURATIONS CAN BE EXPECTED THROUGH THE DAY, WITH IMPROVEMENT IN
THE EVENING. STP
PRELIMINARY POINT TEMP/POPS
TDY FRI SAT SUN MON TUE WED
-------------------------------------------------------
BIL 057 039/068 044/078 051/087 054/090 055/085 054/080
+/T 10/U 00/U 00/U 00/U 11/B 43/T
LVM 053 030/066 039/075 046/082 048/083 049/078 046/073
+/T 10/B 00/U 01/B 01/U 14/T 55/T
HDN 056 036/069 040/080 046/087 050/089 052/086 053/082
+/R 20/U 00/U 00/U 01/U 11/B 32/T
MLS 058 039/068 041/077 048/086 052/090 058/090 057/083
7/R 20/B 00/U 00/U 00/U 10/N 22/T
4BQ 054 039/066 039/075 046/085 052/090 058/088 057/082
8/T 51/B 00/U 00/U 00/U 00/N 22/T
BHK 061 038/064 037/076 044/085 051/090 057/089 057/082
7/T 73/W 00/U 00/U 00/U 00/N 22/T
SHR 046 031/064 034/072 040/082 044/086 046/084 046/080
+/R 50/B 01/U 00/U 00/U 02/T 33/T
BYZ WATCHES/WARNINGS/ADVISORIES
MT...WINTER STORM WARNING IN EFFECT UNTIL 6 PM MDT THIS EVENING
FOR ZONE 67.
WINTER WEATHER ADVISORY IN EFFECT UNTIL 9 PM MDT THIS EVENING
FOR ZONE 171.
WY...WINTER WEATHER ADVISORY IN EFFECT UNTIL 9 PM MDT THIS EVENING
FOR ZONE 198.
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